Turbofan gas turbine engines for powering aircraft conventionally comprise a core engine, which drives a fan. The fan comprises a number of radially extending fan blades mounted on a fan rotor which is enclosed by a generally cylindrical, or frustoconical, fan casing. The core engine comprises one or more turbines, each one of which comprises a number of radially extending turbine blades enclosed by a cylindrical, or frustoconical, casing.
There is a remote possibility with such engines that part, or all, of a fan blade, or a turbine blade, could become detached from the remainder of the fan or turbine. In the case of a fan blade becoming detached this may occur as the result of, for example, the turbofan gas turbine engine ingesting a bird or other foreign object. It is a requirement that the fan blade element is contained within the engine and does not cause damage to an aircraft structure.
The use of containment rings for turbofan gas turbine engine casings is well known. It is known to provide generally cylindrical, or frustoconical, relatively thick metallic containment rings. It is also known to provide generally cylindrical, or frustoconical, locally thickened, isogrid, metallic containment rings. Furthermore it is known to provide strong fibrous material wound around relatively thin metallic casings or around the above mentioned containment casings. In the event that a blade becomes detached it passes through the casing and is contained by the fibrous material.
However, in the event that a blade becomes detached, the blade strikes the metal casing and a significant load is imparted from the main impact region of the metal casing to a flanged interface with an intake casing or rear fan casing.
It is normal practice to transfer the impact loads along the metal casing to the flanged interface joint with the intake or rear fan casing. The flanged interface is bolted together with an array of circumferentially spaced bolts. Under impact by a released fan blade, the flanged joint between the metal casing and the intake or rear fan casing has a tendency to open thus permitting an unwanted and substantial degree of movement of the joint. To counter this, the flanged joint is a substantial structure and is therefore of considerable weight. Furthermore it is a requirement for the bolts to resist the considerable shear force between casings and therefore the bolts are also of substantial number, size and weight. Nevertheless it is known that the integrity of the bolted flange joint can be lost and debris from a blade off event pass through the flanged joint. Furthermore, in order to provide an integral joint, the prior art flanged joint is relatively stiff compared with the rest of the fan casing, so that it resists deflection during impact, and as a result very high local stresses are developed in other parts of the casing, potentially leading to undesirable failure.
EP1277919, of the present applicant, discloses a joint assembly for limiting an extension of the joint in the direction of a load path derived from an impact. The joint assembly comprises a first member having overlapping portion that are arranged generally parallel to one another and are secured together via a conventional bolt assembly disposed through corresponding holes defined therein. One of the overlapping portions further defines, sequentially in the direction of extension, a shear neck, a pocket and a catcher portion. In the event of a worst case impact load the securing means shears through the shear neck and the pocket and is arrested by the catcher portion, thereby the extension of the joint assembly is limited and the joint assembly remains integral.